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From 71b0389fbb31833d827f5f0fec18880c2f602753 Mon Sep 17 00:00:00 2001
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From: Alexander Bokovoy <abokovoy@redhat.com>
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Date: Thu, 19 May 2022 13:52:22 +0300
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Subject: [PATCH] mkhomedir: add support for pre-CVE-2020-10737 behavior
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Pre-CVE-2020-10737 behavior was used to allow creating home directories
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on NFS mounts when non-Kerberos authentication method is in use. This is
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exactly the case where a race condition addressed by the CVE-2020-10737
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fix could have happened. However, there are legit use cases where this
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setup is needed.
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Add '-f' option to mkhomedir helper to activate previous behavior. In
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order to enable it, a change to oddjobd-mkhomedir.conf configuration
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file is needed by explicitly adding '-f' option to the executable file
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definition.
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Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2050079
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Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
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---
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src/mkhomedir.c | 16 +++++++++++++---
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src/oddjobd-mkhomedir.conf.5.in | 9 +++++++++
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2 files changed, 22 insertions(+), 3 deletions(-)
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diff --git a/src/mkhomedir.c b/src/mkhomedir.c
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index be85959..ac813a9 100644
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--- a/src/mkhomedir.c
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+++ b/src/mkhomedir.c
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@@ -53,9 +53,11 @@ static const char *skel;
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static const char *skel_dir;
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static struct passwd *pwd;
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static mode_t override_umask;
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+static int owner_mkdir_first = 0;
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#define FLAG_POPULATE (1 << 0)
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#define FLAG_QUIET (1 << 1)
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+#define FLAG_OWNER_MKDIR_FIRST (1 << 2)
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/* Given the path of an item somewhere in the skeleton directory, create as
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* identical as possible a copy in the destination tree. */
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@@ -158,7 +160,7 @@ copy_single_item(const char *source, const struct stat *sb,
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* target user just yet to avoid potential race conditions
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* involving symlink attacks when we copy over the skeleton
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* tree. */
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- if (status->level == 0) {
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+ if (status->level == 0 && !owner_mkdir_first) {
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uid = 0;
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gid = 0;
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}
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@@ -222,6 +224,9 @@ mkhomedir(const char *user, int flags)
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pwd->pw_dir);
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return HANDLER_INVALID_INVOCATION;
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}
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+ if (flags & FLAG_OWNER_MKDIR_FIRST) {
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+ owner_mkdir_first = 1;
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+ }
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if ((lstat(pwd->pw_dir, &st) == -1) && (errno == ENOENT)) {
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/* Figure out which location we're using as a
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* template. */
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@@ -237,7 +242,7 @@ mkhomedir(const char *user, int flags)
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int res = nftw(get_skel_dir(), copy_single_item, 5,
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FTW_PHYS);
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/* only now give ownership to the target user */
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- if (res == 0) {
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+ if (res == 0 && !owner_mkdir_first) {
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res = chown(pwd->pw_dir, pwd->pw_uid, pwd->pw_gid);
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}
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@@ -317,8 +322,11 @@ main(int argc, char **argv)
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umask(override_umask);
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skel_dir = "/etc/skel";
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- while ((i = getopt(argc, argv, "nqs:u:")) != -1) {
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+ while ((i = getopt(argc, argv, "nqfs:u:")) != -1) {
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switch (i) {
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+ case 'f':
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+ flags |= FLAG_OWNER_MKDIR_FIRST;
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+ break;
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case 'n':
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flags &= ~FLAG_POPULATE;
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break;
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@@ -339,6 +347,8 @@ main(int argc, char **argv)
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break;
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default:
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fprintf(stderr, "Valid options:\n"
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+ "-f\tCreate home directory initially owned by user, "
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+ "not root. See man page for security issues.\n"
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"-n\tDo not populate home directories, "
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"just create them.\n"
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"-q\tDo not print messages when creating "
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diff --git a/src/oddjobd-mkhomedir.conf.5.in b/src/oddjobd-mkhomedir.conf.5.in
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index d7a2429..6e35ad5 100644
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--- a/src/oddjobd-mkhomedir.conf.5.in
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+++ b/src/oddjobd-mkhomedir.conf.5.in
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@@ -10,6 +10,15 @@ directory.
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The mkhomedir helper itself accepts these options:
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.TP
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+-f
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+Restore behavior before CVE-2020-10737 was fixed: create the home directory
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+with user's ownership directly rather than create it as a root and only after
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+populating it change to the user's ownership. The former behavior is insecure
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+but may be used to allow creation of NFS-mounted home directories when
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+non-Kerberos authentication is in use. It is prone for a race condition that
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+could be exploited in the NFS-mounted home directories use case. To avoid
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+CVE-2020-10737, do not use \fB-f\fR option in production environments.
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+.TP
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-q
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Refrain from outputting the usual "Creating home directory..." message when it
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creates a home directory.
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--
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2.37.1
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